Longitudinal determination of total body creatine pool size and skeletal muscle mass in rats by D3‐creatine dilution

Abstract

We recently described a new method to determine total body creatine pool size and skeletal muscle mass based on dilution of an oral dose of D3‐creatine in urinary creatinine. Enrichment of D3‐ creatinine was measured by isotope ratio mass spectrometry. Here we adapt the method to LC‐MS/MS, and investigate the potential for repeated application over time. Rats were given an oral tracer dose of D3‐creatine (1.0 mg/kg body weight) at 10 weeks of age. Urine D3‐creatinine enrichment was determined 72 hr after tracer administration and creatine pool size was calculated. Lean body mass (LBM) measured by quantitative magnetic resonance correlated with creatine pool size (r = 0.92, P = 0.0002). The rate constant for decline in urinary D3‐creatinine enrichment was slow and linear (2.73 ± 0.06 %/day). Subtracting background urinary D3‐creatinine enrichment from the enrichment following a second dose of D3‐creatine at 17 weeks permitted repeat calculations of creatine pool size. As at 10 weeks, 17 week LBM correlated with creatine pool size (r = 0.98, P < 0.0001). In addition, the change in creatine pool size was correlated with the change in LBM during the 7 weeks of somatic growth between measurements (r = 0.96, P < 0.0001). Thus, the LC‐MS/MS‐based D3‐creatine dilution method can be applied repeatedly to measure skeletal muscle mass change in longitudinal studies. These studies were funded by GlaxoSmithKline.